DIFFERENT TYPES OF CASTING PROCESS:

1) Investment casting

2) Permanent mold casting

3) Centrifugal casting

4) Continuous casting

5) Sand casting

Investment casting

Investment casting (known as lost-wax casting in art) is a process that has been practiced for thousands of years, with lost wax process being one of the oldest known metal forming techniques. From 5000 years ago, when bees wax formed the pattern, to today’s high technology waxes, refractory materials and specialist alloys, the castings ensure high quality components are produced with the key benefits of accuracy, repeatability, versatility and integrity.

Investment casting derives its name from the fact that the pattern is invested, or surrounded, with a refractory material. The wax patterns require extreme care for they are not strong enough to withstand forces encountered during the mold making. One advantage of investment casting it that the wax can be reused.

The process is suitable for repeatable production of net shape components, from a variety of different metals and high performance alloys. Although generally used for small castings, this process has been used to produce complete aircraft door frames, with steel castings of up to 300 kg and aluminum castings of up to 30 kg. Compared to other casting processes such as die casting or sand casting it can be an expensive process, however the components that can be produced using investment casting can incorporate intricate contours, and in most cases the components are cast near net shape, so requiring little or no rework once cast.

Permanent mold casting

Permanent mold casting (typically for non-ferrous metals) requires a set-up time on the order of weeks to prepare a steel tool, after which production rates of 5-50 pieces/hr-mold are achieved with an upper mass limit of 9 kg per iron alloy item (cf., up to 135 kg for many nonferrous metal parts) and a lower limit of about 0.1 kg. Steel cavities are coated with a refractory wash of acetylene soot before processing to allow easy removal of the workpiece and promote longer tool life. Permanent molds have a limited life before wearing out. Worn molds require either refinishing or replacement. Cast parts from a permanent mold generally show 20% increase in tensile strength and 30% increase in elongation as compared to the products of sand casting.

The only necessary input is the coating applied regularly. Typically, permanent mold casting is used in forming iron, aluminum, magnesium, and copper based alloys. The process is highly automated.

Sub-types of permanent mold casting

1. Gravity Die Casting.

2. Low pressure die casting.(LPDC)

3. High pressure die casting.(PDC)

Centrifugal casting

Centrifugal casting is both gravity- and pressure-independent since it creates its own force feed using a temporary sand mold held in a spinning chamber at up to 900 N (90 g). Lead time varies with the application. Semi- and true-centrifugal processing permit 30-50 pieces/hr-mold to be produced, with a practical limit for batch processing of approximately 9000 kg total mass with a typical per-item limit of 2.3-4.5 kg.

Industrially, the centrifugal casting of railway wheels was an early application of the method developed by German industrial company Krupp and this capability enabled the rapid growth of the enterprise.

Continuous casting

Continuous casting is a refinement of the casting process for the continuous, high-volume production of metal sections with a constant cross-section. Molten metal is poured into an open-ended, water-cooled copper mold, which allows a 'skin' of solid metal to form over the still-liquid centre. The strand, as it is now called, is withdrawn from the mold and passed into a chamber of rollers and water sprays; the rollers support the thin skin of the strand while the sprays remove heat from the strand, gradually solidifying the strand from the outside in. After solidification, predetermined lengths of the strand are cut off by either mechanical shears or travelling oxyacetylene torches and transferred to further forming processes, or to a stockpile. Cast sizes can range from strip (a few millimeters thick by about five metres wide) to billets (90 to 160 mm square) to slabs (1.25 m wide by 230 mm thick). Sometimes, the strand may undergo an initial hot rolling process before being cut.

Continuous casting is used due to the lower costs associated with continuous production of a standard product, and also increases the quality of the final product. Metals such as steel, copper and aluminium are continuously cast, with steel being the metal with the greatest tonnages cast using this method.

Sand casting

Sand casting is one of the most popular and simplest types of casting that has been used for centuries. Sand casting allows for smaller batches to be made compared to permanent mold casting and a very reasonable cost. Not only does this method allow for manufacturers to create products for a good cost there are other benefits to sand casting such as there are very little size operations. From castings that fit in the palm of your hand to train beds (one casting can create the entire bed for one rail car) it can be done with sand casting. Sand casting also allows for most metals to be cast depending in the the type of sand used for the molds.

Sand casting requires a lead time of days for production at high output rates (1-20 pieces/hr-mold), and is unsurpassed for large-part production. Green (moist) sand has almost no part weight limit, whereas dry sand has a practical part mass limit of 2300-2700 kg. Minimum part weight ranges from 0.075-0.1 kg. The sand is bonded together using clays (as in green sand) or chemical binders, or polymerized oils (such as motor oil.) Sand in most operations can be recycled many times and requires little additional input.

DESIGN CONSIDERATIONS IN DIE CASTING

The critical items to be considered while designing a DIE CASTING DIE are

Ø Shrinkage.

Ø Draft angle.

Ø Selection of Parting Surface.

Ø Number of Cavities.

Ø Feed System.

Ø Cooling System.

Ø Ejection System.

Ø Venting.

SHRINKAGE

Shrinkage is the difference between the dimensions of Die and Die Block. When designing the die it is important to specify the proper material shrinkage in order to achieve a part that meets the dimensional requirement.

Draft Angle

Draft is necessary for the ejection of the parts from the Die. To properly release an die part from the tool, parts are almost always designed with a taper in the direction of die movement. This allows the die part to break free by creating a clearance as soon as die starts to open. Since materials shrink as they cool, they grip cores of the die very tightly. Recommended draft angle is normally 1° with 1/2° on ribs. Some draft angle is better than none and more draft is desirable if the design permits. Where minimum draft is desired, good polishing is recommended and feature depth should not exceed 0.5inch.

PARTING SURFACE

The various factors to be considered while selecting the parting surface are:

§ Shape of the Components.

§ Type of Die.

§ Method of Ejection.

§ Method of Manufacture.

§ Location and Type of Gate.

§ Aesthetics of Die.

In general the parting surface is classified as,

Ø Flat parting surface.

Ø Non-flat parting surface.

Flat parting surface

Parting surface having only one plane is termed as Flat parting surface. The position of the parting surface will therefore be at the top of the Die Casting.

Non-flat parting surface

Parting line which lies on a non-planar or curved surface termed as non-flat parting surface.

NUMBER OF CAVITIES

Number of cavities is decided by,

· Size, Shape and Weight of the Component.

· Ease of Manufacturing.

· Machine capacity.

· Production Quantity.

In this dissertation work the two components are to be planned to accommodate in single tool for ease of manufacture and production wise. Hence 2 cavity to be considered for the design.

FEED SYSTEM

It is necessary to provide a flow-way in an die to connect the nozzle (of the machine) to the each impression. This flow-way is termed as feed system. Normally feed system comprises of

1) Sprue.

2) Runner.

3) Gate.

1) Sprue

The sprue is the channel along which the molten first enters the Die. It delivers the melt from the nozzle to the runner system. The sprue is incorporated in a hardened steel bush which has a seat designed to provide a good seal with the nozzle.

The sprue has to demold easily and reliably and therefore has to be tapered. The taper is generally 2° in most cases.

2) Runner

The runner is the flow path by which the molten travels from the sprue (i.e. the die casting machine) to the gates (i.e. the cavity). To prevent the runner freezing off prematurely, its surface area should be small so as to minimize heat transfer to the die. However, the cross sectional area of the runner should be large so that it presents little resistance to the flow but not so large that the cycle time needs to be extended to allow the runner to solidify for ejection.

The following factors must be considered while designing a runner system.

Ø Cross section of runner.

Ø Size of the runner.

Ø Runner layout.

The different types of runner profiles widely used in mould are

1) Half round.

2) Fully round.

3) Rectangular.

4) Hexagonal.

5) Trapezoidal.

6) Modified trapezoidal.

3) Gate

Gate is the small orifice which connects the runner to the cavity. It has a number of functions. Firstly, it provides a convenient weak link by which the die can be broken off from the runner system. In some dies the degating may be automatic when the mould opens. The gate also acts like a valve in that it allows molten plastic to fill the die but being small it usually freezes off first. The cavity is thus sealed off from the runner system which prevents material being sucked out of the cavity during screw-back.

Gate locations should ensure following conditions:

· Ensure a balanced flow (rapid and uniform filling) in the cavity so that certain areas of the part are not over packed.

· Ensure Die fills under realistic temperatures and pressures.

· Minimize weld lines as much as possible, or position them in non critical areas.

· Prevent "jetting" by positioning the gate so that the material flow is smooth and uniform.

· Avoid air entrapment.

· Gate into the thickest section and direct material flow.

In this tool side/edge gate is selected. The reason for selecting edge gate is as follows,

Ø To fill the cavity sufficiently without any filling problems in the components.

Ø The cross-sectional form is simple and, therefore cheap to machine.

Ø Close accuracy in the gate dimensions can be achieved.

Ø The filling rate of the impression can be controlled relatively independently of the gate seal time.

The edge gate is located so as it should be easily degated and it should not affect the components aesthetic appearance

COOLING SYSTEM

The velocity of heat exchange between the injected Material and the Die is a decisive factor in the economical performance of an Die Casting Die. Heat has to be taken away from the material until a stable state has been reached. The time needed to accomplish this is called cooling time

The easiest method of cooling is to drill holes through the various plates. These holes are being placed near the center of the die and as close to the impression as possible. In many cases these holes are made longer, to allow turbulent flow of fluid so as to achieve economy in cooling. Cooling-channel configurations can be serial or parallel.

The die, however, may consist of areas too far away to accommodate regular cooling channels. Alternate methods for cooling these areas uniformly with the rest of the part involve the use of

· Baffle.

· Bubbler.

· Heat Pipe.

· Heat Rod.

Factors to be considered while designing the cooling

1) Wall thickness of the component.

2) The active area of cooling channel.

3) Position of gate and location of runners.

4) Temperature of the die.

5) Length of cooling hole.

6) Type of cooling.

EJECTION SYSTEM

After the Die has solidified and cooled down, it has to be removed from the die. It would be ideal if gravity could separate the part from cavity or core after die opening. The die is kept in place, however, by undercuts, adhesion and internal stresses and, therefore, has to be separated and removed from the die by ejection system.

Ejection system is usually actuated mechanically by the opening stroke of the die casting machine. If this simple arrangement is not sufficient ejection can be performed pneumatically or hydraulically.

Four factors should be considered in designing the ejection mechanism:

· Shape and geometry of the part.

· Type of material and wall thickness.

· Projected production volume.

· Component position relative to the parting line.

The basic ejection techniques are:

· Pin ejection.

· Blade ejection.

· Sleeve ejection.

· Valve ejection.

· Air ejection.

· Stripper plate ejection.

According to shape, material wall thickness, component position relative to parting plane and aesthetic considerations the pin ejection technique is chosen for the part ejection.

VENTING

The die must be vented in order to release the air that is trapped when the material flows into them. Poor venting can result in short shots, weld lines, burn marks, and high stresses resulting from high packing pressures.

The number of vents in a mold is often limited by the economics of construction. Good part design includes specifying vent location on part prints.

In general, higher melt flow materials must use smaller vents than a low melt flow version of the same material.

Vent Location

Vents can be positioned anywhere along the parting line of the die, particularly at last-to-fill locations. A reasonable guide is to have vents spaced at 25 mm pitch. For blind ribs and bosses, vents may be incorporated into the mold by grinding flat spots along the major axis of an ejector pin or cavity.